Decolorizable ink and printer

ABSTRACT

Decolorizable ink consisting of an ink composition dispersed in a solvent, the ink composition comprising a color former, a developer and a decolorizer, wherein the color former and developer are in a colored state by interaction between them and the decolorizer has a property to dissolve preferentially the developer when the ink composition is melted.

BACKGROUND OF THE INVENTION

The present invention relates to decolorizable ink which can bedecolored after it is printed and a printer using the ink.

In recent years, the amount of various kinds of information hassignificantly increased by spread of office automation, and so the levelof information output has also increased. The information output isrepresented by display output and hard copy output from a printer ontopaper sheets. The display output, however, requires a large scalecircuit board in a display unit. This brings about problems ofportability and cost. Regarding the hard copy output, a large quantityof paper as a recording medium is being consumed with increase in theinformation output amount. Therefore, the hard copy output is expectedto be a problem with respect to conservation of natural resources. Inaddition, recycling of paper sheets once printed by a printer or acopying machine is expensive, since much of a bleaching agent and waterare required for the recycling. Under such a situation, it is consideredto decrease consumption of paper substantially by using decolorizableink to print information on a paper sheet, restoring a blank sheet ofpaper by decoloring the ink, and reusing the paper sheet.

Heretofore, ink which can be decolored on heating has been proposed in,for example, Published Unexamined Japanese Patent Application No.7-81236. The ink includes a color former such as a leuco dye, adeveloper, and a organophosphoric compound having a decoloring power.

When such ink is used, however, decoloring can be done insufficientlyand, as a result, a paper sheet is hard to return to the blank state.For this reason, decolorizable ink cannot have been put into practicaluse.

BRIEF SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to provide inkwhich can give print having a sufficient contrast and can be decoloredby heating, thereby making it possible to recycle paper sheets. It isanother object of the present invention to provide a printer using theink.

Decolorizable ink of the present invention comprises an ink compositiondispersed in a solvent, the ink composition including a color former, adeveloper and a decolorizer, wherein the color former and the developerare in a colored state by interaction between them and the decolorizerhas a property to dissolve preferentially one of the color former andthe developer when the ink composition is melted.

A method of printing with ink and decoloring the ink of the presentinvention comprises steps of preparing a printing sheet consisting of asheet substrate on which an ink composition including a color former anda developer is applied, and thermally transferring the ink compositionto a medium, thereby printing information on the medium with the inkcomposition, and preparing a decoloring sheet consisting of anothersheet substrate on which a composition including a decolorizer isapplied, and thermally transferring the composition including thedecolorizer to the printed medium, thereby decoloring the inkcomposition.

A printer of the present invention comprises a printing mechanism toprint a medium with the above mentioned decolorizable ink and a printingmechanism to print a medium with a normal ink.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic view of a mechanism for decoloring thedecolorizable ink of the present invention;

FIG. 2 is a perspective view of an apparatus for decoloring thedecolorizable ink of the present invention;

FIG. 3 is a schematic view of another mechanism for decoloring thedecolorizable ink of the present invention;

FIG. 4A is a perspective view of an ink-jet printer of the presentinvention, and FIG. 4B is a schematic view of a decoloring mechanismused in the printer;

FIG. 5 is a schematic view of another ink-jet printer of the presentinvention;

FIG. 6 is a perspective view of still another ink-jet printer of thepresent invention;

FIG. 7 is a view illustrating a structure of still another ink-jetprinter;

FIG. 8 is a view illustrating a structure of a copy machine ofelectrophotographic type of the present invention;

FIG. 9A is a sectional view of a sheet on which an ink of the presentinvention is applied, and FIG. 9B is a sectional view of a sheet onwhich a decolorizer of the present invention is applied;

FIG. 10 is a graph showing a relationship between the TPH power and therefractive density obtained in Example 7 of the present invention;

FIG. 11 is a graph showing a relationship between the TPH power and thereflection density obtained in Example 8 of the present invention;

FIG. 12 is a graph showing a relationship between the TPH power and thereflection density obtained in Example 9 of the present invention; and

FIG. 13 is a graph showing a relationship between the TPH power and thereflection density obtained in Example 10 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below.

Decolorizable ink of the present invention uses a color former and adeveloper in combination with a decolorizer to enable decoloring of aprinted medium.

First, functions of basic components used in the decolorizable ink ofthe present invention will be described. A color former is a precursorcompound of a coloring matter which forms colored information such ascharacters and graphics, a developer is a compound which develops acolor former by the interaction (primarily, exchange of an electron orproton) between the developer and the color former, and a decolorizer isa compound having a property that preferentially dissolves one of thecolor former and the developer when they are melted.

When the three-component system is in a solidified state, it is possiblefor the system to assume one of the following two particular states:

(1) A colored state in which the decolorizer is mixed with the colorformer and the developer in an amount that corresponds to theequilibrium solubility, and the excessive color former and developerover the equilibrium solubility are phase-separated from thedecolorizer, with the result that the interaction between the colorformer and the developer is increased to develop a color.

(2) A decolored state in which the decolorizer dissolves a larger amountof the color former and the developer than the equilibrium solubility,with the result that the interaction between the color former and thedeveloper is decreased to lose the color.

Changes between colored and decolored states of the three-componentsystem are effected in accordance with a principle described below. Itis assumed in the following description that, when the above mentionedthree-component system is melted into a fluidized condition, thedecolorizer preferentially dissolves the developer. At room temperature,a condition in which a phase of the color former and the developer isseparated from a phase of the decolorizer is close to equilibrium. Inthis condition, the system is in a colored state, since the color formerand the developer interact with each other. When the three-componentsystem in the colored state is heated up to the melting point or higherto be a fluidized condition, the developer is preferentially dissolvedinto the decolorizer. As a result, the interaction between the developerand the color former is lost, leading to decoloring. When thethree-component system is forcedly solidified by cooling rapidly fromthe molten state, the decolorizer takes the developer into itself in alarge amount exceeding the equilibrium solubility. As a result, thesystem is turned amorphous and colorless at room temperature. Althoughthe amorphous three-component system is under a non-equilibrium state ina relative sense, the amorphous system exhibits a sufficiently long lifeat temperatures not higher than a glass transition point Tg. Therefore,if Tg is not lower than room temperature, the system does not easilyconverted from the amorphous state to the equilibrium state.

Next, compounds used as components of the ink of the present inventionare described below.

The color former used in the present invention includeselectron-donating organic substances such as leucoauramines,diarylphtalides, polyarylcarbinoles, acylauramines, arylauramines,Rohdamine B lactams, indolines, spiropyrans and fluorans.

To be more specific, the color former includes Crystal Violet lactone(CVL), Malakite Green lactone,2-anilino-6-(N-cyclohexyl-N-methylamino)-3-methylfluoran,2-anilino-3-methyl-6-(N-methyl-N-propylamino)fluoran, 3-4-(4-phenylaminophenyl)aminophenyl!amino-6-methyl-7-chlorofluoran,2-anilino-6-(N-methyl-N-isobutylamino)-3-methylfluoran,2-anilino-6-(dibutylamino)-3-methylfluoran,3-chloro-6-(cyclohexylamino)fluoran, 2-chloro-6-(diethylamino)fluoran,7-(N,N-diethylamino)-3-(N,N-diethylamino)fluoran, 3,6-bis(diethylamino)fluoran, γ-(4'-nitroanilino)lactam,3-diethylaminobenzo a!-fluoran, 3-dietylamino-6-methyl-7-aminofluoran,3-diethylamino-7-xylidino-fluoran,3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindole-3-yl)-4-azaphthalide,3-(4-diethylaminophenyl)-3-(1-ethyl-2-methylindole-3-yl) phthalide,3-diethylamino-7-chloroanilinofluoran, 3-diethylamino-7, 8-benzofluoran,3,3-bis(1-n-butyl-2-methylindole-3-yl)phthalide,3,6-dimethylethoxyfluoran, 3,6-diethylamino-6-methoxy-7-aminofluoran,DEPM, ATP, ETAC, 2-(2-chloroanilino)-6-dibutylaminofluoran, CrystalViolet carbinol, Malachite Green carbinol,N-(2,3-dichlorophenyl)leucoauramine, N-benzoylauramine, Rhodamine Blactam, N-acetylauramine, N-phenylauramine,2-(phenyliminoethanedilydene)-3,3-dimethylindoline,N,3,3-trimethylindolinobenzospiropyran,8'-methoxy-N,3,3-trimethylindolinobenzospiropyran,3-diethylamino-6-methyl-7-chlorofluoran,3-diethylamino-7-methoxyfluoran, 3-diethyamino-6-benzyloxyfluoran,1,2-benzo-6-diethyaminofluoran, 3,6-di-p-toluidino-4,5-dimetylfluoran,phenylhydrazide-γ-lactam, and 3-amino-5-methylfluoran. The color formercompounds exemplified above can be used singly or in a combination oftwo or more species. If color formers are selected properly, a varietyof colored states can be obtained, and thus multicolor printing can beattained.

The developer used in the present invention includes acidic compounds,such as phenols, metal phenolates, metal carboxylates, benzophenones,sulfonic acids, sulfonates, phosphoric acids, metal phosphates, acidicphosphoric esters, acidic phosphoric ester metal salts, phosphrousacids, and metal phosphites. The developer compounds can be used singlyor in a combination of two or more species.

The decolorizer used in the present invention should desirably have agood colorlessness in an amorphous state. If the decolorizer is morecolorless and transparent in the amorphous state, a recycled paper sheetis turned white closer to the original paper sheet when the ink on thepaper sheet is decolored. The decolorizer shows such characteristicsshould preferably have a high molecular weight and a small enthalpychange of melting ΔH of the crystal per weight and, thus, should be lowin maximum crystallization velocity (MCV). If the crystal of decolorizerhas a small enthalpy change of melting ΔH, the heat energy required formelting the crystal is decreased, which is desirable in regard to energysaving. From the above considerations, it is desirable to use as thedecolorizer a compound having a bulky molecular skeleton close to aspherical form such as a steroid skeleton. In order to increasesolubility of the developer in the decolorizer, it is desirable that thedecolorizer has a high affinity with the developer. Therefore, thedecolorizer should desirably be a compound having, for example, analcoholic hydroxyl group. From a view point of a storage stability ofthe three-component system in a decolored state, a glass transitionpoint Tg of the three-component system should be not lower than roomtemperature (25° C.), and preferably be not lower than 50° C. In orderto satisfy the above condition, the glass transition point Tg of thedecolorizer should also be not lower than room temperature (25° C.), andpreferably be not lower than 50° C. On the other hand, thecrystallization temperature of a decolorizer is in the range of theglass transition point Tg to the melting point Tm of the three-componentsystem. Therefore, in order to accelerate decoloring, the glasstransition point Tg of a decolorizer should preferably be not higherthan 150° C.

As a preferable decolorizer that satisfies above conditions, thefollowing compounds classified in the groups (a) to (c) are enumerated.

(a) Sterol compounds: Specific examples are choresterol, stigmasterol,pregnenolone, methylandrostenediol, estradiol benzoate, epiandrostene,stenolone, β-sitosterol, pregnenolone acetate, β-chorestanol,5,16-pregnadiene-3β-ol-20-one, 5α-pregnen-3β-ol-20-one, 5-pregnen-3β,17-diol-20-one 21-acetate, 5-pregnen-3β, 17-diol-20-one 17-acetate,5-pregnen-3β, 21-diol-20-one 21-acetate, 5-pregnen-3β, 17-dioldiacetate, rockogenin, thigogenin, esmiragenin, heckogenin, diosgeninand their derivatives. These decolorizers can be used singly or in acombination of two or more species. Particularly preferable decolorizerwhich can give a stable decolored state includes methylandrostenediol,heckogenin, rockogenin, thigogenin, diosgenin and esmiragenin.

When a three-component system containing the decolorizer selected fromthe above group in an amorphous state is heated to a temperature higherthan a glass transition point, a diffusion velocity of a developer israpidly increased and a motion of phase separation between the developerand the decolorizer is accelerated in a direction of returning to aequilibrium. If the three-component system heated to a temperaturehigher than the crystallizing temperature and lower than the meltingpoint is then slowly cooled down to room temperature, the system reachesto a stable phase separated state closer to a equilibrium, at which thesystem returns to a colored state. Therefore, the three-component systemincluding the decolorizer of the (a) group can repeat reversible changesbetween colored and decolored states. In this sense, the decolorizerclassified in the (a) group is sometimes referred to, hereinafter, as a"reversible decolorizer". A rewritable recording medium which utilizessuch reversible changes has been proposed. However, the presentinvention has an object to provide decolorizable ink whose color isremoved after printed, and therefore the reversibility between coloredand decolored states is not substantially required in the presentinvention with the exception of some special applications.

(b) Cholic acid, lithocholic acid, testosterone, cortisone and theirderivatives: Specific examples are cholic acid, methyl cholate,lithocholic acid, methyl lithocholate, hydroxycholic acid, methylhydroxycholate, testosterone, methyltestosterone,11α-hydroxymethyltestosterone, and hydrocortisone. Among them, compoundshaving two or more hydroxyl groups are especially preferred.

The decolorizer of the (b) group, compared to that of the (a) group, hasa stronger affinity to the developer when they are melted, in otherwords, has a very high compatibility thereto. In addition, thedecolorizer of the (b) group has a higher inclination of beingamorphous, and therefore a phase separation is hard to occur even afterthe three-component system is solidified. In this sense, the decolorizerclassified in the (b) group is sometimes referred to as a "compatibledecolorizer" hereinafter. For this reason, the three-component systemincluding the decolorizer of the (b) group can maintain a stablerdecolored state.

(c) Non-aromatic cyclic compounds of a five-membered or larger ringhaving one or more hydroxyl groups: The decolorizer of the (c) groupshould have a melting point of 50° C. or higher. Specific examples arealicyclic monohydric alcohols such as cyclododecanol; alicyclic dihydricalcohols such as 1,4-cyclohexandiol, 1,2-cyclohexandiol and1,2-cyclododecandiol; saccharides and their derivatives such as glucoseand saccharose; alcohols having a ring structure such as1,2:5,6-diisopropylidene-D-mannitol.

The decolorizer of the (c) group functions effectively when it is usedtogether with the decolorizer of the (a) group, although it may be usedsingly. That is, the decolorizer of the (c) group has a strong affinitywith the decolorizer of the (a) group, and therefore a phase separationis hard to occur even after the system is solidified. In this sense, thedecolorizer of the (c) group is sometimes referred to as a "phaseseparation inhibiting decolorizer" or "phase separation inhibitor"hereinafter. The system including the decolorizer of the (c) group canalso maintain a stabler decolored state.

Embodiments of the decolorizable ink of the present invention and amethod of using the same will be described below.

In one embodiment of the decolorizable ink of the present invention, anink composition comprising a color former, a developer and a decolorizerthat is in a colored state is dispersed in a solvent. Preferable mixingratio of the color former, the developer and the decolorizer is asfollows. It is desirable to use the developer in an amount of 0.1 to 10parts by weight, preferably 1 to 2 parts by weight, relative to 1 partby weight of the color former. If the amount of the developer is smallerthan 0.1 parts by weight, coloring of the ink by the interaction betweenthe color former and the developer becomes insufficient. If the amountof the developer exceeds 10 parts by weight, it becomes difficult todecrease sufficiently the interaction between these compounds. It isdesirable to use the decolorizer in an amount of 1 to 200 parts byweight, preferably 10 to 100 parts by weight, relative to 1 part byweight of the color former. If the amount of the decolorizer is smallerthan 1 part by weight, changes between the colored and decolored statescannot occur easily. If the amount of decolorizer exceeds 200 parts byweight, coloring of the ink becomes insufficient.

A solvent used in the decolorizable ink of the present invention ispreferably water. Although the ink composition may be dispersed in thesolvent by the help of a surfactant, it is preferred that microcapsuleshaving a structure in which the ink composition is covered by a polymershell are dispersed in the solvent. In order to improve weatherresistance and ultraviolet resistance, it is preferable to add aultraviolet absorber to the ink composition contained in themicrocapsules or to use a ultraviolet absorbing polymer as a shellmaterial of microcapsule. In preparing microcapsules, it is desirable touse the polymer in an amount of 0.01 to 100 parts by weight, preferably0.1 to 5% parts by weight, relative to 1 part by weight of thedecolorizer.

The concentration of the ink composition in the solvent is preferably inthe range of 0.1 to 10 wt %, although the concentration varies dependingon purposes. If the concentration is smaller than 0.1 wt %, sufficientprint density cannot be obtained. If the concentration exceeds 10 wt %,the viscosity of the ink becomes too high to use, for example, as inkfor a ink-jet printer. The ink composition dispersed in the solventshould preferably be in the form of particles of sub-micron in size notincluding particles of 10 μm or more.

The decolorizable ink of the present invention can be used as ink forball-point pens and fountain pens, a printer ink for an ink-jet printer,and printing ink for screen printing and typographic printing on variouskinds of paper. Moreover, the decolorizable ink is used for printing ona sheet for a overhead projector. The ink can be easily decolored bysimply heating after it is printed. That is, since the ink contains thedecolorizer that preferentially dissolves one of the color former andthe developer, the color of the ink can be almost completely removed bysuppressing the interaction between the color former and the developerwhen the ink is melted and then solidified. Therefore, a paper sheetwith a blank surface can be restored. There are no needs for a bleachingagent and water for recycling of paper, and therefore recycling cost isdecreased. At the same time, decrease in paper consumption contributesto conservation of forest resources.

The decolorizable ink of the present invention can be applied to uses inwhich decoloring of ink is simply required without reusing paper sheets.For example, the ink is applicable to processing post cards in postoffices, wherein the addresses handwritten on post cards are read withan image reader to obtain bar code corresponding to the reading in apost office, bars are then printed on the post cards with the ink of thepresent invention, the post cards are classified with a machineaccording to the bar code, the classified post cards are sent to anotherpost offices, the post cards are sorted in order of postal delivery ineach post office, and then the bars are decolored by heating. In such amanner, post cards can be delivered without any trace of the processingin the post offices. In a manner similar to this, the decolorizable inkof the present invention can generally be used for physicaldistribution. In this purpose, substances other than paper may beprinted.

Next, an apparatus used for decoloring the decolorizable ink of thepresent invention and an ink-jet printer using the decolorizable ink aredescribed below.

An apparatus used for decoloring the decolorizable ink of the presentinvention to recycle paper sheets may be in any form, as long as theapparatus has a mechanism capable of heating and rapid cooling a papersheet. A specific example of such an apparatus is a thermal head havinga heating resistor. A thermal bar, which is one kind of thermal head,having a heating resistor capable of heating the paper sheet across alarge area can be used. In this case, a heating area on a paper sheetmay be adjusted by controlling current in the thermal bar so as todecolor only a desired area of the paper sheet. If a heating apparatussuch as a heat roller is used, a large quantity of printed paper can bedecolored for recycling. In the case where printing on the surface of anarticle not having sheet-like form is to be decolored, the surface ofthe article is heated with an infrared lamp or by hot air and thenrapidly cooled by cold air or with a heat sink having a large heatcapacity.

For example, as shown in FIG. 1, a thermal bar 13 whose heating area isadjusted to a proper area is located between a pair of feed rollers 11,11 and a pair of transport rollers 12, 12, and a paper sheet 10 alreadyprinted with the decolorizable ink of the present invention is passedthrough the thermal bar 13. When electric current is made to flowthrough the thermal bar 13, heating up to 300° C. can be effectedinstantly. The paper sheet 10 is heated with the thermal bar 13, andthen comes out of the apparatus to be cooled rapidly, so that the printon the paper sheet can be decolored. FIG. 2 shows a decoloring apparatus(recycling apparatus) 21 of a hand-feed type which incorporates thedecoloring mechanism of FIG. 1.

FIG. 3 shows a mechanism with which decoloring of print on a paper sheetand recycling of the paper sheet can be carried out in a more reliablemanner than that in the mechanism of FIG. 1. As shown in FIG. 3, inaddition to the mechanism of FIG. 1, a pair of pressing rollers 14, 14is disposed to smooth wrinkles on the paper sheet, and also a UV source15 and a humidifier 16 are disposed between the transport rollers 12, 12and the pressing rollers 14, 14. In this mechanism, the coloring matteris decomposed completely to be colorless on exposure by radiation fromthe UV source 15, the wrinkles on the paper sheet 10 can effectively besmoothed by the pressing rollers 14, 14 after proper humidity is givenby the humidifier 16. When such a mechanism is used, the number ofrepeating use of the same paper sheet can be increased. Degradation ofthe paper sheet does not become a problem, if a high-grade paper sheetsuch as plastic-coated paper is used for a special purpose.

A printer using the decolorizable ink of the present invention is mostpreferably an ink-jet printer. When ink containing a reversibledecolorizer is used, it is especially preferable to use an ink-jetprinter that injects ink without any heating. Accordingly, the mostpreferable means for injecting the ink is one that uses vibration of apiezoelectric element or supersonic waves. If ink containing adecolorizer of the (b) group and the (c) group or ink containingcomponents all having high Tgs is used, a bubble-jet printer in whichink is heated may be used.

FIGS. 4A and 4B show an ink-jet printer having the decoloring mechanismof FIG. 1. In this printer 31, paper sheets to be printed are stored inthe feed tray 32. A paper sheet is printed by way of a printingmechanism similar to that of a common ink-jet printer and finally comesout from the outlet 33. Upon recycling, a printed paper sheet 10 isinserted through the inlet 34, the paper sheet 10 travels through thedecoloring mechanism equipped with the feed rollers 11, transportrollers 12 and the thermal bar 13 as shown in FIG. 1, and finally thepaper sheet 10 is received into the recovery tray 35. In this case, theprinting tray 32 and the recovery tray 35 are in the same shape so as tobe exchangeable.

FIG. 5 shows an ink-jet printer capable of printing on both new papersheets and printed paper sheets. In this printer 41, trays 42, 43 forrespectively accommodating new and printed paper sheets are mounted on aside of the printer 41. The tray 42 is also used as a tray forrecovering recycled paper after the printed information on the printedpaper is decolored. A new paper sheet is fed from the tray 42, printedwith a printer head 45 while passing through a printing mechanism of acommon ink-jet printer, and then discharged. On the other hand, aprinted paper sheet is fed from the tray 43, decolored with the thermalbar 13, and then recovered in the tray 42 or fed again toward theprinter head 45 so as to be printed and then discharged. Switchingbetween both transport courses is conducted by a switching mechanism 44.

FIG. 6 shows an ink-jet printer having a head for printing with thedecolorizable ink of the present invention and another head for printingwith a normal ink. In this printer 51, the head 52 for printing with thedecolorizable ink and the head 53 for printing with the normal ink areequipped. Printing information is instructed to the printer 51 from anapplication software executed on the computer 61. An image of a papersheet to be printed is displayed on the computer display. Input to afixed information region 62 and a variable information region 63 isconducted on the display. In the printer 51, both heads 52 and 53 forthe decolorizable ink and the normal ink are actuated according to theinstructions from the application software, thereby carrying outprinting on a paper sheet 50 as instructed. With this type of printer,only variable information possibly be rewritten can be repeatedlydecolored and printed again many times.

FIG. 7 shows an ink-jet printer having a decoloring mechanism consistingof a heat roller. In this printer 71, paper sheets placed in thecassette 72 in the bottom are fed by a transport roller, printed by theprinter head 73 and discharged through the outlet 74. When a printedpaper is recycled, the printed paper sheet is fed through the inlet 75,decolored by heating with the heat roller 76, and then dischargedthrough another outlet 77.

In the present invention, toner for electrophotographic prepared bypulverizing a composition containing a color former, a developer, adecolorizer and a binder may be used to decolor a printed paper sheet.In the electrophotographic application, toner transferred on the papersheet is fixed by heating, and therefore it is preferable to use acompatible or phase-separation inhibiting decolorizer.

FIG. 8 shows a copy machine of electrophotographictype having adecoloring mechanism consisting of a heat roller. In this copy machine81, paper sheet are accommodated in the cassette 82 in the bottom. Thephotosensitive drum 83 locating in the central portion of the machine isirradiated with light from the LED head 84, thereby forming a latentimage on the surface of the drum 83. To the drum 83, charged toner 86 isattached with the developing apparatus 85 by an electrostatic force toform a toner image. A paper sheet is, in a timely manner with thedevelopment, transported between the drum 83 and the transfer roller 87,thereby transferring the toner to the paper sheet. The toner transferredon the paper sheet is fixed by the fixing roller 88 and then dischargedthrough the outlet 89. When a printed paper sheet is recycled, the papersheet is fed through the inlet 90, heated by a heat roller 91 to decolorand then discharged through the outlet 92. Temperatures of the fixingroller 88 and the heat roller 91 are so set, for example, 140° C. and190° C., that the temperature of the heat roller 91 is higher than thatof the fixing roller 88.

In the copy machine of electrophotographic type, printing tonercomprising a color former and a developer and decoloring tonercomprising a decolorizer may be used.

Components of the ink of the present invention can be applicable to athermal printer. In this case, a sheet (or ribbon) for printing and asheet (or ribbon) for decoloring are preferably used to effect printingand decoloring. The sheet or ribbon is made of a heat-resistant resinsuch as polyethyleneterephthalate (PET).

For example, an ink composition comprising a color former and adeveloper is applied on a sheet substrate to prepare a printing sheet.The ink composition may be applied together with wax in order tofacilitate transfer to the paper. Another composition including adecolorizer is applied on another sheet substrate to prepare adecoloring sheet. Using a thermal printer, the ink composition on theprinting sheet is thermally transferred to a paper sheet to effectprinting and, when necessary, the composition including the decolorizeron the decoloring sheet is thermally transferred to the printed papersheet to effect decoloring. As to the decolorizer, those categorized inthe compatible decolorizer or the phase-separation inhibitingdecolorizer are preferred. In the decoloring operation, a power of thethermal printer head is raised so as to operate at a higher temperatureas compared with that in the printing operation.

FIG. 9A shows an example of a printing sheet, and FIG. 9B shows anexample of a decoloring sheet. The printing sheet 100 in FIG. 9A has astructure that the release layer 103, the ink layer 104 and thelubricant layer 105 are formed on the PET sheet 101 on the back of whichthe back coating layer 102 is formed. Note that, the release layer 103and the lubricant layer 105 are not necessarily required. The decoloringsheet 200 in FIG. 9B has a structure that the release layer 203, thebarrier layer 204 and the decolorizer layer 205 are formed on the PETsheet 201 on the back of which the back coating layer 202 is formed.With the barrier layer 204 inserted, the decolorizer can be preventedfrom spreading wide over a medium. Note that, the release layer 203 andthe barrier layer 204 are not necessarily required.

In the above case, it may be used an ink composition in colored statewhich comprises a color former, a developer and a decolorizer having arelatively low glass transition point. When the ink composition isthermally transferred to a paper sheet and the printed paper sheet istemporarily decolored by heating and then the paper sheet is left atroom temperature, the decolored printing can be restored to be visible.Such a function is applicable, for example, to judgment of storageconditions of fresh foods to be refrigerated. That is, if a label onwhich warning is printed with the above ink composition is attached onthe package of fresh foods and then the label is decolored, the warningappears on the label when the package is left at room temperature.

In addition, when it is desired that an ink composition has areversibility, a phase-separation accelerator, which is a compound thataccelerates phase separation of a color former and a developer from adecolorizer, may be added in order to raise a coloring speed. Preferredphase-separation accelerator is a low molecular weight organic compound,which is highly crystallizable, having a long chain alkyl group witheight or more carbon atoms and a polar group such as OH, CO and COOH.Examples of phase-separation accelerator are linear higher monohydricalcohols, linear higher polyhydric alcohols, linear higher fatty acids,linear higher polyvalent fatty acids, esters and ethers thereof, linearhigher fatty acid amides and linear higher polyvalent fatty acid amides.

To be more specific, the phase-separation accelerator includes: linearhigher monohydric alcohols such as 1-docosanol, 1-tetracosanol,1-hexacosanol and 1-octacosanol; linear higher polyhydric alcohols suchas 1,12-dodecanediol, 1,12-octadecanediol, 1,2-tetradecanediol and1,2-hexadecanediol; linear higher fatty acids such as behenic acid,1-docosanic acid, 1-tetracosanic acid, 1-hexacosanic acid and1-octacosanic acid; linear higher polyhydric fatty acids such asdodecanedioic acid and 1,12-dodecanedicarboxylic acid; linear higherketons such as stearone; linear higher fatty acid alcohol amides such asisopropanolamide stearate, isopropanolamide behenate and hexanolamidebehenate; and linear higher fatty acid dioldiesters such asethyleneglycol dilaurate, catechol dilaurate and cyclohexanedioldilaurate. These compounds can be used singly or in combination of twoor more species. Examples of a mixture which can be used as thephase-separation accelerator are ester-base waxes, alcohol-base waxesand urethane-base waxes.

Combinations of components applied on a printing sheet and a decoloringsheet are modified from the above mentioned combinations. For example, aprinting sheet applied with an ink composition comprising a colorformer, a developer and a phase-separation inhibiting decolorizer (or acompatible decolorizer) and a decoloring sheet applied with acomposition including a reversible decolorizer may be used. Further,only a printing sheet applied with an ink composition comprising a colorformer, a developer, a reversible decolorizer and a phase-separationinhibiting decolorizer (or compatible decolorizer) in a state where thereversible decolorizer and the phase-separation inhibiting decolorizer(or compatible decolorizer) are phase-separated may be used.

In the present invention, two kinds of printing method may be usedtogether. For example, a paper sheet is printed with toner including acolor former and a developer by electrophotography and then adecolorizer may be thermally transferred from a decoloring sheet to theprinted paper sheet for decoloring.

Further, printing and decoloring may be carried out in the followingmanner. Upon printing, first toner including a color former and adeveloper is transferred on a heat-resistant sheet byelectrophotography, and then the first toner on the heat-resistant sheetis thermally transferred to a paper sheet. Upon decoloring, second tonerincluding a decolorizer is transferred on a heat-resistant sheet byelectrophotography, and then the second toner on the heat-resistantsheet is thermally transferred to the printed paper sheet. In this case,too, decoloring is carried out at a higher temperature than printing. Inthis method, partial correction can be made in a printed region. Innormal electrophotography, since a paper sheet is entirely heated in thefixing process, if a part of printed paper sheet is decolored andsubsequently re-printed and then a different part of the re-printedpaper is decolored, the re-printed part is likely to be decolored in thefixing process. On the contrary, in the above method, correction can berepeated two or more times, since there is no need for the fixingprocess.

EXAMPLES

Examples of the present invention will be described below.

Example 1

One part by weight of Crystal Violet lactone (CVL) as a color former,two parts by weight of 2,4,4'-trihydroxybenzophenone as a developer and10 parts by weight of methylandrostenediol as a decolorizer werehomogeneously mixed in a mixture solvent of toluene and cyclohexane, andthen the solvent was evaporated off to obtain powder. After the powderwas heated up to 150° C., it was cooled down to room temperature in 10minutes to develop blue color. One gram of the powder was put into 50 ccof water added with 0.5 grams of sodium stearate as surfactant, and theink composition powder was pulverized to a particle size of a micronorder in a pulverizer to prepare ink with dispersed fine powder therein.

Using the ink thus prepared, characters were written on a paper sheet ofA4 size with pen. After drying, a thermal bar set at 200° C. was passedabove the paper sheet in two seconds. As a result, the characterscompletely disappeared and a blank paper sheet was obtained. Whenwriting and decoloring were repeated 30 times, also a blank paper sheetwas finally obtained. The optical density (OD) of the blank paper sheetfinally obtained was 0.15.

Example 2

One part by weight of ETAC as a color former, one part by weight of2,4,4'-trihydroxybenzophenone as a developer, five parts by weight ofheckogenin as a decolorizer were homogeneously mixed in a mixturesolvent of toluene and cyclohexane, and then the solvent was evaporatedoff to obtain powder. After the powder was heated up to 150° C., it wascooled down to room temperature in 10 minutes to develop blue color. Onegram of the powder was put into 50 cc of water added with 0.5 grams ofsodium stearate as surfactant, and the ink composition powder waspulverized to a particle size of a micron order in a pulverizer toprepare ink with dispersed fine powder therein.

The ink was used in an ink-jet printer equipped with a piezoelectricelement (MJ800C manufactured by Epson Co.), and printing was carried outon a paper sheet of A4 size. After the printing, a thermal bar waspassed above the paper sheet to decolor the printed information. Whenthe recycled paper was stored at 60° C. for 30 hours, the printedinformation did not appear again, which revealed that the recycled papersheet had a good storage stability. After the paper sheet was printedagain, the printed surface was irradiated with light at 5000 lux for 300hours. In this case, decrease in printing density was not higher than10%.

Example 3

One part by weight of PSD-HR (available from Nippon Soda Co. Ltd.) as acolor former, one part by weight of α, α,α'-tris(4-hydroxyphenyl)-1-ethyl-4-isoproylbenzene as a developer and 20parts by weight of pregnenolone as a decolorizer were melted to form amixture, and then the mixture was gradually cooled to obtain a red solidproduct. The product dissolved in an aqueous solution containing 8 wt %of gum arabic was pulverized with a ball mill to a particle size of theorder of 10 μm so that the powder was dispersed in the aqueous solution.The dispersed product was mixed with an aqueous solution of gelatin at40° C., the mixture was stirred for 1 hours, and then water was addeddropwise while stirring to dilute the mixture. Next, an aqueous solutioncontaining 10 wt % of acetic acid was added to adjust pH of the mixtureto be 3.9, and then 37% formalin was added to adjust pH of the mixtureto be 7.0. The liquid thus prepared was cooled to 5° C., left at roomtemperature for 3 days, and then the liquid was subjected to acentrifugal separator to separate microcapsules. In this way,microcapsules for red color were prepared. The microcapsules weredispersed in water at a concentration of 8 wt % to prepare ink.

The ink was used in a ink-jet printer equipped with a piezoelectricelement (MJ800C, manufactured by Epson Corp.), and printing was carriedout on a paper sheet of A4 size. After the printing, a thermal bar waspassed above the paper sheet to decolor the printed information. Whenthe recycled paper was stored at 60° C. for 30 hours, the printedinformation did not appear again, which revealed that the recycled papersheet had a good storage stability. After the paper sheet was printedagain, the printed surface was irradiated with light at 5000 lux for 300hours. In this case, decrease in printing density was not higher than10%.

Example 4

Microcapsules obtained in a similar method to that of the example 3 wasdispersed in water at a concentration of 15 wt % to prepare ink fortypographic printing.

A paper sheet of A4 size was printed with this ink in a commontypographic printing process. Thereafter, a heat roller kept at 200° C.was made to contact with the sheet for two seconds to decolor theprinted information. When printing and decoloring were repeated 30times, also a blank paper sheet was finally obtained. The opticaldensity (OD) of the blank paper sheet finally obtained was 0.15.

Example 5

Microcapsules were prepared in a similar method to that the example 3except that one part by weight of Crystal Violet lactone (CVL) as acolor former, two parts by weight of 2,4,4'-trihydroxybenzophenone as adeveloper and 20 parts by weight of pregnenolone as a decolorizer wereused. The microcapsules were dispersed in water at a concentration of 8wt % to prepare ink.

This ink was used in a ink-jet printer equipped with a piezoelectricelement (MJ800C manufactured by Epson Corp.) to print on a paper sheetof A4 size. After the printing, a thermal bar was passed above the paperto decolor the printed information. When the recycled paper was storedat 60° C. for 30 hours, the printed information did not appear again,which revealed that the recycled paper sheet had a good storagestability. After the paper sheet was printed again, the printed surfacewas irradiated with light at 5000 lux for 300 hours. In this case,decrease in printing density was not higher than 10%.

Example 6

Two parts by weight of a colored composition consisting of PSD-150(available from Nippon Soda Co. Ltd.) as a color former and2,3,4,4'-tetrahydroxybenzophenone as a developer at a ratio of 1 to 1,seven parts by weight of D-glucose as a decolorizer, three parts byweight of gelatin as a binder, and small amounts of isopropyl alcoholand a mildew inhibitor were added to water to prepare ink with a solidcontent of 10%. The color of the ink was black.

A paper sheet was printed with the ink with an ink-jet printer, and thenthe printed paper sheet was dried. When the paper sheet was heated bymeans of a thermal printer head (TPH) at a power of 0.35 mJ/dot or more,the printed information was able to be decolored. The decolored statewas kept for 1000 hours at 40° C. without any change.

Examples 7 to 11

In the following Examples 7 to 11, 4.5 μm-thick PET film back-coatedwith thermosetting silicone-acrylic resin at a thickness of about 0.2 μmwas used as a sheet substrate to prepare a printing sheet and adecoloring sheet. Printing and decoloring were carried out by using athermal printer head with a pixel density of 8 dot/mm at a recordingvelocity of 10 ms/L.

Example 7

One part by weight of Crystal Violet lactone (CVL) as a color former,one part by weight of propyl gallate as a developer, 15 parts by weightof wax as a thermal transferring agent and 3 parts by weight ofpolystyrene as a binder resin were mixed followed by thermally meltingto prepare an ink composition, and then the melt was applied on asurface of a sheet substrate with a bar coater at a thickness of about 2μm to prepare a printing sheet.

One part by weight of 1,2-cyclohexanediol and one part by weight ofmethyl cholate as a decolorizer were mixed followed by thermally meltingto prepare a decolorizer composition, and then the melt was applied on asurface of another sheet substrate by a bar coater at a thickness ofabout 3 μm to prepare a decoloring sheet.

Printing, decoloring and re-printing were carried out in the followingmanner with changing a TPH power, during which the reflection densitiesof a paper sheet were measured. The results are shown in FIG. 10.

When the printing sheet was put on a paper sheet and the ink compositionwas thermally transferred on the paper sheet by means of TPH, printingwas effected stably at a power of 0.15 mJ/dot or more.

When the decoloring sheet was put on the printed paper sheet and thedecolorizer composition was thermally transferred on the paper sheet bymeans of TPH, decoloring was effected at a power of 0.3 mJ/dot or more.This decolored state was maintained even for longer than 300 hours at40° C.

When the printing sheet was put on the decolored paper sheet and the inkcomposition was thermally transferred again on the paper sheet by meansof TPH, re-printing was effected at a power in the range of 0.15 to 0.2mJ/dot, but re-printing was not able to be effected at a power of 0.3mJ/dot or more.

Example 8

One part by weight of CVL as a color former, one part by weight of2,4,4'-trihydroxybenzophenone as a developer, five parts by weight ofmetylandrostenediol as a reversible decolorizer, five parts by weight of1-docosanol as a phase-separation accelerator and three parts by weightof styrene-methacrylic acid copolymer as a binder were mixed followed bythermally melted to prepare an ink composition, and then the melt wasapplied on a surface of a sheet substrate by means of a bar coater at athickness of 2 μm to prepare a printing sheet. A decoloring sheet wasprepared in a similar manner to that of the example 7.

Printing, reversible decoloring, complete decoloring and re-printingwere carried out in the following manner with changing a TPH power,during which reflection densities of a paper sheet were measured. Theresults are shown in FIG. 11.

When the printing sheet was put on a paper sheet and the ink compositionwas thermally transferred on the paper sheet by means of TPH, printingwas effected stably at a power of 0.15 mJ/dot or more.

When the printed portion on the paper sheet was heated by means of TPH,the printed portion was decolored at a power of 0.35 mJ/dot or more.When the decolored paper sheet was left at room temperature, the printedcharacters were restored to the originally printed state after 72 hourshad passed.

When the decoloring sheet was put on the printed paper sheet and thedecolorizer composition was thermally transferred on the paper sheet bymeans of TPH, decoloring was effected at a power of 0.35 mJ/dot or more.This decolored state was maintained even for longer than 1000 hours at60° C. without any change.

When the printing sheet was put on the decolored paper sheet and the inkcomposition was thermally transferred again on the paper sheet by meansof TPH, re-printing was able to be effected at a power in the range of0.15 to 0.25 mJ/dot, but re-printing was not able to be effected at apower of 0.35 mJ/dot or more.

Example 9

One part by weight of ETAC (available from Yamada Chemical Co. Ltd.) asa color former, one part by weight of 2,4,4'-trihydroxybenzophenone as adeveloper, five parts by weight of methyl lithocholate as a compatibledecolorizer and three parts by weight of polystyrene as a binder weremixed at a temperature where methyl lithocholate is not melted toprepare an ink composition, and then the ink composition was applied ona surface of a sheet substrate by means of a bar-coater at a thicknessof 2 μm to prepare a printing sheet.

β-Sitosterol as a reversible decolorizer and mixed alcohols containingmainly myricyl alcohol as a phase-separation accelerator were mixed at aratio of 2 to 1 by weight followed by thermally melting to prepare adecolorizer composition, and then the melt was applied on a surface ofanother sheet substrate at a thickness of 2 μm by means of a bar-coaterto prepare a decoloring sheet.

Printing, decoloring and re-printing were carried out in the followingmanner with changing a TPH power, during which the reflection densitiesof the paper sheet were measured. The results are shown in FIG. 12.

When the printing sheet was put on a paper sheet and the ink compositionwas thermally transferred on the paper sheet, printing was effectedstably at a power of 0.10 mJ/dot or more. When the printed portion ofthe paper sheet was heated by means of TPH, it was not able to decolorthe printed characters.

When the decoloring sheet was put on the printed paper sheet and thedecolorizer composition was thermally transferred on the paper sheet bymeans of TPH, decoloring was effected at a power of 0.3 mJ/dot or more.The decolored state was not changed even after 300 hours had elapsed at40° C.

When the printing sheet was put on the decolored paper sheet and the inkcomposition was thermally transferred again on the paper sheet,re-printing was effected at a power in the range of 0.15 to 0.2 mJ/dot.However, re-printing was not effected at a power of 0.3 mJ/dot or more.

Example 10

One part by weight of RED 40 (available from Yamada Chemical Co., Ltd.)as a color former, one part by weight of2,3,4,4'-tetrahydroxybenzophenone as a developer and 10 parts by weightof 1,2-cyclohexanediol as a phase-separation inhibiting decolorizer weremixed by melting, and then the solid mixture was pulverized into finepowder with a particle size of 10 μm or less.

On the other hand, one part by weight of RED 40 as a color former,2,3,4,4'-tetrahydroxybenzophenone as a developer and 10 part by weightof cholesterol as a reversible decolorizer were mixed by melting, andthen the solid mixture was pulverized into fine powder with a particlesize of 10 μm or less.

These two kinds of powder were mixed at a ratio of 1 to 1 by weight andthe mixture was dispersed in an aqueous solution containing 5% gelatinat a solid content of 20% to prepare ink. The ink was then applied on asurface of a sheet substrate with a coating weight of about 3 g/m² bymeans of a bar-coater to prepare a printing sheet. In this example, nodecoloring sheet was used.

FIG. 13 shows a relationship between the TPH power and the reflectiondensity. When the printing sheet was put on a paper sheet and the inkcomposition was thermally transferred on the paper sheet by means ofTPH, printing was effected at a power in the range of 0.10 to 0.3mJ/dot, but printing was not effected at a power exceeding 0.3 mJ/dot.

When the printed portion of the printed paper was heated by means ofTPH, it was unable to decolor the printed portion at a power in therange of 0.10 to 0.3 mJ/dot, but it was able to decolor the printedportion at a power exceeding 0.3 mJ/dot. The decolored state wasmaintained without any change for 300 hours at 40° C.

Example 11

One part by weight of CVL as a color former, one part by weight ofpropyl gallate as a developer and 10 parts by weight of choresterol as areversible decolorizer were melted to prepare a composition. Using acomposition, microcapsules with a gelatin shell were prepared. Threeparts by weight of the microcapsules and one part by weight of wax as athermal transferring agent were mixed and the mixture was applied on asheet substrate by means of a hot-melt coater to prepare a printingsheet.

The printing sheet was put on a paper sheet and then the ink compositionwas thermally transferred on the paper sheet at a power of 0.15 mJ/dotby means of TPH to print such a phrase as "STORAGE POSSIBLY IMPROPER".The printed paper was heated at 60° C. to decolor the characterstemporarily.

When the decolored paper was stored in a refrigerator at 10° C., theprinted characters did not appear even after 240 hours had passed.However, when the decolored paper was left in a room at 25° C., theprinted characters appeared after 10 hours had passed. Similarly, in thecondition of 35° C., the characters appeared after two hours had passed.Therefore, the ink of the present invention can be applicable as athermal history sensor in the use such as quality control of freshfoods.

A temperature at which printing with an ink of the present invention isrestored, after it is printed and then decolored temporarily, iscontrollable by selecting material having a proper glass transitionpoint. Glass transition point Tg and melting point Tm of typicalreversible decolorizers are shown in Table 1. Moreover, coloring speedof ink in the vicinity of the above mentioned temperatures werecontrolled by addition of a phase-separation accelerator or selection ofa proper developer.

    ______________________________________    No.     Decolorizer       Tg      Tm    ______________________________________    1       1,2:5,6-Diisopropylidene-D-                               2° C.                                      106° C.            Mannitol    2       Pregrenolone Acetate                              15° C.                                      140° C.    3       Δ-Androstene-3, 17-dione                              22° C.                                      130° C.    4       Cholesterol       28° C.                                      137° C.    5       Lanosterol        33° C.                                      131° C.    6       5α-Pregnan-3β-ol-20-one                              37° C.                                      182° C.    7       Pregnenolone      44° C.                                      183° C.    8       Estradiol Benzoate                              50° C.                                      188° C.    9       5α-Pregnan-3β, 17-diol                              53° C.                                      196° C.            diacetate    10      Methylandrostendiol                              65° C.                                      197° C.    11      Heckogenin        80° C.                                      250° C.    12      Rockogenin        92° C.                                      201° C.    ______________________________________

Example 12

One part by weight of CVL as a color former, one part by weight ofpropyl gallate as a developer, 10 parts by weight of cholesterol as areversible decolorizer, three parts by weight of wax as a thermaltransferring agent, five parts by weight of polystyrene as a binder, andsmall amounts of hydrophobic quartz filler in the form of ultra-finepowder and an antistatic agent were melted and then solidified. Thesolid product was crushed and further pulverized into fine particleswith an average size of 10 μm to prepare toner. A decoloring sheet wasprepared in a similar manner to that of the example 7.

The toner was fed to a copy machine (FC210 manufactured by Cannon Inc.)and characters were copied on a paper sheet. They were fixed stably.

When the decoloring sheet was put on the copied paper and then thedecolorizer was thermally transferred by means of THP, decoloring waseffected at a power of 0.3 mJ/dot or more. The decolored state wasunchanged even after 300 hours had elapsed at 40° C.

Example 13

One part by weight of ETAC (made by Yamada Chemical Co., Ltd.) as acolor former, one part by weight of 2,4,4'-trihydroxybenzophenone as adeveloper, 10 parts by weight of heckogenin as a reversible decolorizer,three parts by weight of mixed alcohols rich in myricyl alcohol as aphase-separation accelerator, five parts by weight ofstyrene-methacrylic acid copolymer as a binder, and small amounts ofhydrophobic quartz filler in the form of ultra-fine powder and anantistatic agent were melted and then solidified. The solid product wascrushed and further pulverized into fine particles with an average sizeof 10 μm to prepare toner. A decoloring sheet was prepared in a similarmanner to that of the example 7.

The toner was fed into a copy machine (FC210 manufactured by Canon Inc.)and characters were copied on a paper sheet. The characters were fixedon the paper sheet stably.

When the copied portion of the paper sheet was heated by means of TPH ata power of 0.35 mJ/dot or more, it was able to decolor the copiedcharacters. The decolored characters did not appear at room temperature,but they appeared at 40° C. after 3 weeks had passed, and at 135° C. atan elapsed time of 0.2 seconds.

When the decoloring sheet was put on the copied paper and then thedecolorizer was thermally transferred by means of THP, decoloring waseffected at a power of 0.35 mJ/dot or more. The decolored state wasunchanged even after 1000 hours had elapsed at 60° C.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

We claim:
 1. Decolorizable ink consisting of an ink compositiondispersed in a solvent, said ink composition comprising a color former,a developer and a decolorizer,wherein said color former and developerare in a colored state by interaction between them and said decolorizerhas a property to dissolve preferentially one of said color former anddeveloper when the ink composition is melted, and wherein saiddecolorizer is selected from the group consisting of cholic acid.lithocholic acid, testosterone, cortisone and their derivatives.
 2. Thedecolorizable ink according to claim 1, wherein said developer iscontained in an amount of 0.1 to 10 parts by weight relative to one partby weight of said color former.
 3. The decolorizable ink according toclaim 1, wherein said decolorizer is contained in an amount of 1 to 200parts by weight relative to one part by weight of said color former. 4.The decolorizable ink according to claim 1, wherein said solvent iswater.
 5. The decolorizable ink according to claim 1, wherein the inkcomposition is dispersed in the solvent in the form of microcapsules. 6.The decolorizable ink according to claim 1, wherein a concentration ofthe ink composition in the solvent is in the range of 0.1 to 10 wt %.